true, it may not be a breach of contract. However, using your example, it would be like if you went to that store, lets say it's a gas station, and you bought gas there every month, for 2 years. However, they decided for whatever reason not to charge you tax for all those times, then all of a sudden 2 years later you get a letter saying, "hey, you know all that gas you bought in the past 2 years? yeah, umm, we're gonna need to collect tax on it."

The way I understood your post, the tax didn't exist until recently? So it's not the situation where the seller has been omitting collecting the tax, but rather the City introduced a new tax, and retailers can absorb 100% of the tax, or pass it on and collect 100% from their customers, or anything in between.